Design and synthesis of novel liquid crystalline molecules are a focus of research in the field of liquid crystal displays (LCDs) ^{[1, 2, 3, 4]}. Liquid crystals (LCs),which exhibit high values of birefringence (Δn),are potentially useful for incorporation into electro-optic devices,including various display configurations,such as supertwisted nematic liquid crystal displays (STN-LCDs),scattering-type polymer-dispersed liquid crystal displays (PDLCDs),reflective liquid crystal displays,and spatial light modulators ^{[5, 6, 7]}. With the rapid development of information technology,design and synthesis of highly birefringent liquid crystalline materials has become an important issue due to the fact that highly birefringent liquid crystals are urgently needed for fast third-order non-linear switching devices ^[8]. In addition,a number of new applications of highly birefringent liquid crystals have been reported,for example,in reflectors,polarizers,organic light-emitting diodes (OLEDs) and for improving viewing angles in compensation films ^{[9, 10, 11, 12, 13]}.

In the past few years,many highly birefringent materials have been obtained by introducing highly polarizable end-groups and by increasing the molecular π-electron conjugation length [14- 16]. Naphthylethyne skeletons with cyano or isothiocyanato terminating groups have also been reported as candidates for high birefringence LC compounds ^[17]. However,the optical properties of UV absorption and selective reflection have not yet been reported. In the present study,our objective has been to obtain a series of LC molecules with high Δn values based on naphthylethyne and the optical properties of UV absorption spectrum and selective reflection.

The ¹H NMR and ¹³C NMR spectra were recorded on an AVANCZ 400 spectrometer at 298 K using CDCl₃ as a solvent and tetramethylsilane (TMS) as an internal standard. FT-IR spectra were recorded on a Perkin-Elmer spectrophotometer using a powdered sample on a KBr plate. UV-vis absorption spectra were recorded on a UV-3100 spectrophotometer. Polarizing optical micrographs (POM) were recorded on an Olympus BX51equipped with a hot stage (LinkamLK-600PM). The Δn was evaluated according to the guest-host method ^[8] by using an Abbe refractometer (2WAJ),and differential scanning calorimetry (DSC) was recorded on a Perkin-Elmer Pyris 6.

Reagents were purchased from commercial sources (Aldrich) and used without further purification,apart from triethylamine (TEA) and tetrahydrofuran (THF),which were distilled under argon before use.

The synthetic routes of the target liquid crystal compounds are shown in Scheme 1.

Scheme 1

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Scheme 1.The synthesis routes and reagents: a, C5H11Br, K2CO3, DMF; b, Et3N, THF, Pd(PPh3)4, CuI; c, K2CO3, MeOH; d, CSCl2, CaCO3.

2-Bromo-6-pentyloxy-naphthalene: 2-Bromopentane (4.5 g, 30 mmol) was added dropwise through a syringe to a suspension of 6-bromo-2-naphthol (3.3 g,15 mmol) and K₂CO₃ (6.2 g, 45 mmol) in DMF (30 mL). The solution was stirred at 90 ℃ for 4 h. After the reaction completed,the solvent was removed by vacuum distillation. The crude product was purified by column chromatography (silica gel,hexane: CH₂Cl₂ = 4:1) to give the target product (3.94 g,90%) as a white solid. ¹H NMR (400 MHz,CDCl₃) :δ 7.98 (s,1H),7.69 (d,1H,J= 8.0 Hz),7.54 (d,1H,J= 8.0 Hz),7.51 (d,2H,J = 8.0 Hz),7.18 (d,1H,J= 8.0 Hz),7.11 (s,1H),4.03 (t,2H), 1.88 (m,2H),1.44 (m,2H),1.29 (m,2H),0.96 (t,3H); FTIR (cm^-1): ν 3108,2925,2854,1604,1483,1466,1345,1216,1184,997,882, 803,720; MALDI-TOF-MS (dithranol): m/z: calcd. for C₁₅H₁₇BrO: 292.03,found: 293.14[MH]⁺.

Trimethyl-(6-pentyloxynaphthalen-2-ylethyny)-silane: 2-Bromo- 6-pentyloxynaphthalene (2.92 g,10 mmol) and TMSA (1.17 g, 12 mmol) were dissolved in 100 mL dry triethylamine (Et3N): tetrahydrofuran (THF) (1:1),then,Pd(PPh₃)4 (0.28 g,0.24 mmol) and CuI (0.09 g,0.48 mmol) were added. The reaction mixture was stirred at room temperature for 8 h under an Ar atmosphere. Upon completion,the solvent was removed in vacuo and the crude material purified by column chromatography on silica gel (hexane:CH₂Cl₂ = 2:1) to give the target compound (2.57 g,83%) as a white solid. ¹H NMR (400 MHz,CDCl₃) :δ 8.02 (s,1H),7.72 (d,1H,J= 8.0 Hz),7.67 (d,1H,J= 8.0 Hz),7.53 (d,2H,J= 8.0 Hz), 7.16 (d,1H,J= 8.0 Hz,),7.10 (s,1H),4.04 (t,2H),1.88 (m,2H),1.44 (m,2H),1.28 (m,2H),0.96 (t,3H).0.08 (s,9H); FTIR (cm^-1): ν 3109, 2924,2853,2185,2174,1604,1483,1466,1345,1212,1184,995, 882,800,724; MALDI-TOF-MS (dithranol): m/z: calcd. for C₂₀H₂₆OSi: 310.17,found: 310.47.

2-Ethynyl-6-pentyloxynaphthalene: K₂CO₃ (2.76 g,20 mmol) was added to a solution of trimethyl-(6-pentyloxy-naphthalen-2- ylethyny)-silane (2.17 g,7 mmol) in 80 mL THF:MeOH (7:3). The reaction mixture was stirred at room temperature for 3 h. Upon completion,the mixture was filtered by suction filtration. The solvent was removed in vacuo and the crude product was purified by column chromatography (CH₂Cl₂ as the eluent) to give the target compound (1.51 g,91%) as a white solid. ¹H NMR (400 MHz, CDCl₃):δ 8.01 (s,1H),7.73 (d,1H,J= 8.0 Hz),7.68 (d,1H,J= 8.0 Hz), 7.56 (d,2H,J= 8.0 Hz),7.17 (d,1H,J= 8.0 Hz,),7.13 (s,1H),4.09 (t,2H),3.06 (s,1H),1.89 (m,2H),1.44 (m,2H),1.28 (m,2H),0.96 (t,3H); FTIR (cm^-1): ν 3297,3109,2924,2853,2185,2108,2045, 1604,1483,1466,1345,1212,1184,995,882,800,724;MALDI-TOFMS (dithranol): m/z: calcd. for C₁₇H₁₈O: 238.13,found: 238.32 .

4-(6-Pentyloxynaphthalene-2-ylethynyl)-benzonitrile (NTP5OCN): The synthesis of NTP5OCN used the same method as described for the preparation of 1-bromo-6-pentyloxy-naphthalene by the reaction between 2-ethynyl-6-pentyloxy-naphthalene and 4-Iodo-benzonitrile. The product was a white solid,yield: 78%. ¹H NMR (400 MHz,CDCl₃):δ 8.04 (s,1H),7.78 (d,1H,J= 8.0 Hz), 7.70 (d,1H,J= 8.0 Hz,),7.59 (d,1H,J= 8.0 Hz),7.56 (d,2H, J= 8.0 Hz),7.24 (d,2H,J= 8.0 Hz),7.21 (d,1H,J= 8.0 Hz),7.17 (s, 1H),4.04 (t,2H),1.87 (m,2H),1.44 (m,2H),1.26 (m,2H),0.96 (t, 3H),¹³C NM (100 MHz,CDCl₃) d 158.3,134.7,132.1,132.0,131.9, 129.4,128. 0,128.5,128.3,127.0,120.0,118.6,116.9,111.2,106.6, 94.7,87.5,68.2,28.9,28.3,22.5,14.1; FTIR (cm^-1): ν 3109,2924, 2853,2243,2239,2185,2108,2045,1604,1483,1466,1345,1212, 1184,995,882,800,724. MALDI-TOF-MS (dithranol): m/z: calcd. for C₂₄H₂₁NO: 339.15,found: 339.42.

4-(6-Pentyloxynaphthalene-2-ylethynyl)-trifluoromethyl-phenyl (NTP5OCF3): The synthesis of NTP5OCF3 used the same method as described for the preparation of A with a yield of 76% as a white solid. ¹H NMR (400 MHz,CDCl₃):δ 8.01 (s,1H),7.73 (d,1H, J= 8.0 Hz),7.68 (d,1H,J= 8.0 Hz),7.62 (d,1H,J= 8.0 Hz),7.54 (d,2H,J= 8.0 Hz),7.21 (d,2H,J= 8.0 Hz),7.17 (d,1H,J= 8.0 Hz), 7.13 (s,1H),4.09 (t,2H),1.89 (m,2H),1.44 (m,2H),1.28(m,2H), 0.96 (t,3H),¹³C NM (100 MHz,CDCl₃) d 158.1,134.5,131.8,131.7, 129.9,129.4,128.8,128.3,127.4,126.9,125.3,125.2 (q,J= 220 Hz), 119.9,117.2,106.6,92.6,87.6,68.1,28.9,28.3,22.5,14.1; FTIR (cm^-1): ν 3109,2924,2853,2185,2108,2045,1604,1483,1466, 1345,1212,1184,995,882,800,724 cm^-1. MALDI-TOF-MS (dithranol): m/z: calcd for C₂₄H₂₁F₃O: 382.15,found: 382.42.

4-(6-Pentyloxynaphthalene-2-ylethynyl)-phenylamine: The synthesis used the same method as described for the preparation of A with a yield of (73%) as a yellow solid. ¹H NMR (400 MHz, CDCl₃):δ 7.96 (s,1H),7.74 (d,1H,J= 8.0 Hz),7.69 (d,1H,J= 8.0 Hz), 7.53 (d,1H,J= 8.0 Hz),7.31 (d,2H,J= 8.0 Hz),7.16 (d,1H, J= 8.0 Hz),7.10 (s,1H),6.58 (d,2H,J= 8.0 Hz),4.04 (t,2H),4.0 (s, 2H),1.87 (m,2H),1.44 (m,2H),1.26 (m,2H),0.96 (t,3H); FTIR (cm^-1): ν 3451,3447,3109,2924,2853,2185,2108,2045,1604, 1483,1466,1345,1212,1184,995,882,800,724. MALDI-TOF-MS (dithranol): m/z: calcd. for C₂₃H₂₃NO: 329.18,found: 329.43.

2-(4-Isothiocyanato-phenylethyny)-6-pentyloxy-naphthalene (NTP5ONCS): 4-(6-Pentyloxynaphthalene-2-ylethynyl)-phenylamine (1.27 g,4 mmol) was dissolved in 30 mL CHCl₃,then CaCO₃ (0.8 g,8 mmol) and CSCl₂ (0.68 g,6 mmol) were added. The solution was stirred at 30 ℃ for 3 h. Upon completion,the mixture was washed with water and extracted with CH₂Cl₂. The solvent was removed in vacuo,and the crude product was purified by column chromatography (CH₂Cl₂) to give the target compound (1.18 g,82%) as a white solid. ¹H NMR (400 MHz,CDCl₃):δ 7.98 (s,1H),7.74 (d,1H,J= 8.0 Hz),7.69 (d,1H,J= 8.0 Hz),7.53 (d,1H, J= 8.0 Hz),7.51 (d,2H,J= 8.0 Hz),7.23 (d,2H,J= 8.0 Hz),7.16 (d,1H,J= 8.0 Hz),7.10 (s,1H),4.04 (t,2H),1.87 (m,2H),1.44 (m,2H),1.26 (m,2H),0.96 (t,3H). ¹³C NM (100 MHz,CDCl₃) d 158.1,136.5,134.4,132.7,131.5,130.7,129.4,128.8,128.4,126.9, 125.8,122.7,119.9,117.5,106.6,92.2,88.0,68.1,29.0,28.3,22.5, 14.1.; FTIR (cm^-1): ν 3109,2924,2853,2185,2108,2045,1604, 1483,1466,1345,1212,1184,995,882,800,724. MALDI-TOF-MS (dithranol): m/z: calcd. for C₂₄H₁₁NOS: 361.06,found: 361.41.

The phase transitions of compounds were investigated by DSC on heating at a scanning rate of 10 ℃ min1,and these are listed in Table 1. The thermostability of compounds was investigated by STA; the curves are list in Fig. 1. The compounds all exhibited mesophases,and their melting points are above 100 ℃,which is ascribed to the presence of a triple bond and naphthalene moieties that effectively increase the length of π-electron conjugation in the molecular structure. In addition,it is interesting to note that the melting point and clearing point of the compounds vary considerably,and the liquid crystal phase range of NTP5OCF₃ is lower than those of NTP5OCN and NTP5ONCS. This pattern is similar to the thermostability of the compounds. The result is attributed to the highly polarizable end-groups. Cyano and isothiocyanato have strong polarization which can effectively increasing the molecular π-electron conjugation length.

Fig. 1

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Fig. 1.The TG curves of the synthesized compounds.

Birefringence (Δn) is one of the most important parameters of liquid crystal materials. After demonstrating the liquid crystalline phase structures of all the compounds,we focused our attention on their birefringent (Δn) properties. Δn was evaluated from composites containing a specific amount of synthesized compounds in BHR-32100-100(commercial liquid crystal material,Δn is 0.23) using an Abbe refractometer at room temperature. In order to achieve an indication of the validity of the measured value,the Δn values of various weight ratios of the compounds in BHR- 32100-100 were studied. Taking NTP5ONCS as an example,the content of NTP5ONCS in BHR-32100-100 ranged from 2% to 8%. Fig. 2 illustrates the dependence of the Δn values on the content of NTP5ONCS in BHR-32100-100. A linear relationship between the Δn values of the composite and the content of NTP5ONCS in BHR- 32100-100 is shown in Fig. 2. Δn values of the synthesized compounds could be determined from composites containing 8 wt% of the compound in BHR-32100-100 using Fig. 2. Δn data for the synthesized compounds are summarized in Table 1. Δn is increased by exchanging for highly polarizable end-groups. It is well known that high Δn values may be achieved by introducing strong polar end-groups or by increasing the molecular conjugation length. Accordingly,NTP5ONCS has the highest Δn value in this series.

Table 1

Table 1 Phase transition temperatures,phase structures,and Δn of compounds.

Table 1 Phase transition temperatures,phase structures,and Δn of compounds.

Fig. 2

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Fig. 2.Dependence of Dn values of composites on the content of NTP5ONCS in BHR-32100-100.

Typically,highly birefringent compounds are solid at room temperature. Thus,we measured the UV absorption spectra from a dichloromethane solution. Fig. 3 shows the UV absorption spectra of the three compounds we synthesized. The compounds of NTP5ONCS and NTP5OCN have a longer absorption tail than that of NTP5OCF₃,which is chiefly because the former compounds have a longerπ-electron conjugation than the latter,accordingly they also have higher Δn values as mentioned above. The maximum absorption edge of the three compounds was 380 nm. The use of these materials would be suitable for optical display applications because such applications do not have an absorption band in the visible region ^[18].

Fig. 3

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Fig. 3.The UV-visible spectra of the synthesized compounds

A chiral nematic liquid crystal (N^*-LC) is formed when a nematic LC (N-LC) is doped with a chiral dopant ^[19]. N^*-LC has a periodic variation in refractive index,which can be used for optical filtering of circularly polarized incident light of the same handedness as its helix. In order to investigate the effect of the synthesized compounds on the reflection behavior of the N^*-LC,10% weight ratio mixtures of the synthesized compounds in the N^*-LC were studied at the same content of R811 (15 wt%) (As lists in Table 2). Fig. 4 illustrates the dependence of the spectra of N^*-LC samples on the presence of the three compounds, respectively. Without the synthesized compounds in this study, the Δλ was about 126 nm for sample A,whereas it is about 148, 169,and 186 nm for sample B,C,and D,respectively. The reflection band became significantly broader after adding the compounds with high birefringence (Δn). According to the Bragg equation Δλ = ΔnP,when the concentration of the chiral dopant remained unchanged (e.g. the pitch P is a certain value),Δλ was linearly related to Δn only. Thus,it can clearly be seen that the reflection band becomes much broader with increasing Δn of the compounds. The changes in Δλ were due to a change in Δn of the N^*-LC ^[20]. The reflection bandwidth followed a similar sequence with a descending order of Δn of D,C,B.

Table 2

Table 2 The composition and content of the N*-LC.

Table 2 The composition and content of the N*-LC.

Fig. 4

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Fig. 4.Dependence of the reflection band on the samples A-D.

A series of highly birefringent (Δn) compounds based on a naphthylethyne core with cyano,isothiocyanato and trifluoromethyl as the terminal groups has been successfully synthesized and the effect of their different highly polarizable end-groups was investigated. Results showed that highly birefringent compounds had relatively high melting and clearing temperatures due to the strong conjugate structure. Compared with trifluoromethyl group, cyano and isothiocyanato moieties have stronger polarizability, which can effectively increase the molecular π-electron conjugation length. In other words,there is a positively correlated relationship between the birefringent characteristic and the π-electron conjugation length of molecules. We also studied the selective reflection behavior of the compounds,which demonstrated that higher Δn produced a broader reflection band in the N^*-LC.

This work was supported by the Sino-American Cooperative Project of Chinese Ministry of Science and Technology (No. 2013DFB50340),the National Science Foundation of China (No. 51173155),the Major Project of Beijing Science & Technology Program (No. Z121100006512002),the National Natural Science Foundation of China (No. 51103010),and the Doctoral Fund of Chinese Ministry of Education (No. 20120001130005).